1. Field of the Invention
The present invention relates to storage support systems for storage area networks and more particularly to a system and method of virtualizing SCSI semantics from a host computer or server onto a non-SCSI transport medium of a storage area network.
2. Description of the Related Art
For the last couple of decades, the number of computer systems has grown exponentially and the capabilities of the average computer system to assimilate and process data has doubled every eighteen months. These two facts have created a spectacular rise in the demand for raw data storage capacity to support the growing capabilities of computers to assimilate and process data. The demand to add more data storage capability to computer networks, service more users, and provide data security in the form of data backup storage has been unrelenting. The trend in recent years has been to provide a separate data storage network, or storage area network (SAN), for storing data for a network of computers.
The primary advantage of a SAN is the ability of a group of computer users to pool their data without using a local area network (LAN) for tasks like transferring data files and providing backup of stored data files. For example, on a LAN when one station on the LAN generates a 500 MB video file, a second station can access the 500 MB video file if the first station simply transfers the read/write permissions for the video data file to the second station. A very large data file that might take hours to transfer on a LAN can be transferred to a SAN in minutes or seconds. The use of a SAN as opposed to a LAN frees valuable bandwidth on the LAN for client applications and provides scalability not found in the more traditional approaches to data storage such as connecting read/write servers directly to the LAN. Other advantages of a SAN configuration over a LAN configuration include computer clustering, topological flexibility, fault tolerance, high availability, and remote management. Because of the clear advantages of a SAN over a LAN, SAN approach to data storage has been implemented with success between a variety of high data access devices like file servers, backup servers, database servers, and shared storage arrays.
The primary storage interface for servers deployed in SAN systems is a parallel Small Computer System Interface (SCSI). While various forms of parallel SCSI's exist, the predominant form of parallel SCSI has been termed SCSI-2. SCSI-2 introduced enhanced SCSI device data transfer methodologies while maintaining the asynchronous transfer of data, instructions, and messages from earlier SCSI implementations.
Due to certain limitations inherent in a parallel SCSI, many designers of network systems are starting to turn from a traditional parallel SCSI to a non-SCSI transport medium such as a Fibre Channel interconnect medium. As computers and peripheral devices have continued to improve in performance capability, the limitations imposed by a parallel SCSI became apparent. Higher performance processors demanded higher throughput of the subsystem used to store data. A Fibre Channel interconnect medium has substantially addressed these concerns. A Fibre Channel interconnect medium has a serial interface which supports increased bandwidth, additional connectivity, longer distance operation, and greater flexibility.
Several topologies are defined for a Fibre Channel interconnect medium including Fabric, point-to-point, and the Arbitrated Loop. The Fabric topology is a switched connection that allows multiple pairs of devices to exchange messages simultaneously, creating a high aggregate bandwidth. The point-to-point topology is probably the simplest, consisting of a single connection between two devices. The most popular topology is the Arbitrated Loop (FC-AL) in which the out-bound cable of one device becomes the in-bound cable for the next, allowing for the connection of up to 126 devices at a time.
All of the topologies share a single coding scheme and framing protocol. A Fibre Channel interconnect medium currently sends information in frames containing a 24-byte header followed by up to 2118 bytes of data. At most, handshaking occurs after the transmission of every frame of data, unlike the synchronous transfer mode in a SCSI protocol in which a `sender` is allowed to perform a fixed number of transfers before the `sender` (target or initiator) stalls waiting for an acknowledgment. Like the traditional parallel bus SCSI protocol, every transaction has an initiator and a target and makes use of the SCSI command set semantics.
Current operating system environments are designed to be able to communicate with SCSI-2 devices attached via the traditional parallel SCSI bus. The SCSI-2 standard is currently in the process of being updated to produce a SCSI-3 standard. One of the key features of SCSI-3 is an attempt to separate the command protocol from dependencies on the physical interconnection medium, thus allowing for SCSI-3 command and data flow over a serial bus such as a Fibre Channel interconnect medium. Current operating system environments, however, have bene unable to handle many aspects of supporting a SCSI-3 device on a non-parallel bus because of the semantic differences between parallel and non-parallel busses. In addition, so far as is known, the notion of `bridging` SCSI-2 servers across a serial bus environment or non-SCSI transport medium to SCSI-2 devices such as tape drives, storage medium changers, printers, scanners, optical memory devices, CD-ROM drives or hard disk drives is not provided for by an established or developing standard.
The first SCSI-3 devices available are likely to be SCSI-3 hard disk drives. Certain other devices which are available as SCSI-2 devices may not be offered as SCSI-3 devices for some time. Since Fibre Channel interconnect medium protocol (SCSI-FCP) is currently a SCSI-3 protocol, the use of Fibre Channel interconnect medium has precluded proper connectivity of SCSI-2 servers-command initiators to target SCSI-2 devices.